It is sometimes desirable to know the internal temperature of integrated circuits. The integrated circuit temperature can be used to improve circuit operation by compensating signals that change from nominal operation when the chip temperature changes.
A memory device, such as a NAND flash memory, requires various voltages for programming and reading memory cell data. Programming a cell includes biasing the cell control gate with a programming voltage until the cell is at a desired threshold voltage. The cell is verified with a sense amplifier by applying that threshold voltage to the cell to determine if the cell turns on and conducts. If the cell does not turn on, it has not been programmed to the desired threshold.
During operation of an integrated circuit, the temperature varies due to both ambient temperature as well as the electrical operation of the integrated circuit. The temperature change can cause a change in the nominal operating characteristics of a memory cell. For example, a threshold voltage of 1V at room temperature may turn into a threshold voltage of 900 mV as the chip temperature increases. The change in voltage levels can have an impact on reading, programming, and verifying operations that are expecting a certain voltage.
For example, FIG. 1 illustrates a plot of a threshold voltage distribution, Vt. This graph shows typical effects of temperature on the Vt distribution.
The dependence of the decision edge of a sense amplifier on the temperature affects the distribution width of the Vt through the spread of the program verify operation. The program verify operation may be performed at different temperatures such as T1 and T2. If the program algorithm provides a distribution width W, a programming algorithm performed at temperature T1 results in the first distribution 101 that is W wide and starts at pgm_vfy1. If the programming algorithm performs at temperature T2, a second distribution 103 that is W wide starts at pgm_vfy2. The total distribution 104 after the two program operations will be a distribution 104 that is D wide where Wtot≧W+pgm_vfy2−pgm_vfy1. FIG. 1 illustrates the program verify spread 106 resulting from pgm_vfy2−pgm_vfy1.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for temperature compensation of signals in an integrated circuit.